Recent content by A. Neumaier

A Hamiltonian is defined only up to a constant energy shift. The condition fixes this constant by requiring that that the ground state (vacuum) has energy zero. No. The delta function is in the denominator! This is sloppy notation, What is meant is that after multiplication with the...

A.E. Allahverdyan, R. Balian, and T.M. Nieuwenhuizen, Teaching ideal quantum measurement, from dynamics to interpretation, Comptes Rendus Physique 25 (2024), 251--287. https://www.researchgate.net/publication/380862732

That's why Schlosshauer wrote a thick book about it. Indeed, a derivation requires much more that what I had discussed above - namely an analysis that tells which environmental states are appropriate for a given small system (and its embedding into the environment) to ensure that decoherence...

Yes.

Yes. More precisely, collapse is the (not strictly permitted) reinterpretation of the reduced density matrix as an ensemble of pure density matrices. It requires beyond decoherence a mechanism for producing ''unique outcomes'' - the unsolved part of the measurement problem. This is discussed...

Let me give a more detailed exposition of decoherence, based on the book M. Schlosshauer, Decoherence and the quantum-to-classical transition, Springer, New York 2007. The first occurrence of the term 'decoherence' in the main text is on p.6, which gives the closest to a definition that one...

Decoherence is the decay of the off-diagonal elements in the reduced density matrix. Please read the Wikipedia article I cited, or the book by Schlosshauer. The reduced density matrix reproduces exactly the statistics of all observables of the small system. It has an exact nonmarkovian dynamics...

This is correct, but both involve irreversibility and hence violate unitarity of the subsystem dynamics. From Wikipedia: ... while the collapse converts the pure initial state into a projected eigenstate determined by what is measured. In both cases, unitarity is violated. Only the universe as...

No. Decoherence is a process that turns coherent wave functions (superpositions of basis states) into incoherent mixtures (ensembles of basis states). This is inconsistent with the unitary evolution of wave functions. Decoherence is what is obtained from a unitary process for a large system by...

No. The external field is part of the Hamiltonian, not part of the wave function. Thus your assumption is meaningless. But if the wave function of your system does not have arguments that model the macroscopic equipment with which the system is handled, there is already enough decoherence to...

MWI is surely not the mainline view. The world is explained by quantum mechanics, no matter which interpretation you choose.

It is your limitation, but not your fault. On the contrary, Newtonian mechanics and special relativity are incapable of explaining the world as we observe it -- unless we also take quantum mechanics into account!

It would be better to use terminology that means the same for everyone. Otherwise misunderstandings are almost unavoidable! No. Adding an external field changes the unitary dynamics to another unitary dynamics, hence does not create decoherence or collapse. The latter is created exclusively by...

This is not the case. Curved spacetime changes a quantum system by adding an external gravitation field. It is not a ''disruption'' but the same kind of change as adding an external electromagnetic field would have (a standard procedure in quantum mechanics), except that the gravitational field...

Semiclassical gravity is consistent in the thermal interpretation, as is any mixed quantum-classical dynamics following standard patterns. (See Section 7.8 of my book.) Thus there is no absolute need for quantizing gravity. However, since all other quantum-classical dynamical systems studied...